Telecommunication line protection devices are used to protect telephone line and similar telecommunication transmission and switching equipment from power-line contact, electrical strikes and the electromagnetic pulses carried across the telecommunication lines, such as created by a lightning strike or similar electromagnetic event that induces voltages in the telecommunication line. A telecommunication line protector, e.g., a surge protector, is typically required for each telephone or telecommunications subscriber line.
Most tip/ring (T/R) type POTS (Plain Old Telephone Service) circuits use a line card access switch (LCAS) to connect battery feed current and ringing to the T/R conductors. To support long T/R loops, a ringing voltage of 95 to about 100 volts RMS is superimposed on the negative battery feed, resulting in voltages as high as −200 volts or +100 volts on the ring lead. A protection device such as a surge protector is typically set to clamp around 250 volts to about 270 volts to protect the LCAS from unexpected surges resulting from lightning strikes and similar sources. This surge protector or other protection device, however, typically does not turn on during normal operation. A line card access switch typically uses a field effect transistors (FETs) having a breakdown voltage of around 300 volts, which is near the value of any surge protector setting. When undesired transient potentials approach the breakdown voltage of the field effect transistors in the line card access switch, then the field effect transistors are often stressed, thereby reducing the reliability of the line card access switch. Therefore, there is a desire to improve the reliability of a line card access switch due to voltage stress on the field effect transistors.